Light water reactors (“LWRs”) can include pressurized water reactors (“PWRs”) and boiling water reactors (“BWRs”). In a PWR, for example, the reactor core includes a large number of fuel assemblies, each of which is composed of a plurality of elongated fuel elements or rods. The fuel rods each contain fissile material, such as uranium dioxide (“UO2”), usually in the form of a stack of nuclear fuel pellets; although, annular or particle forms of fuel are also used. The fuel rods are grouped together in an array which is organized to provide a neutron flux in the core sufficient to support a high rate of nuclear fission, and thus, the release of a large amount of energy in the form of heat. A coolant, such as water, is pumped through the core in order to extract some of the heat generated in the core for the production of useful work. Fuel assemblies vary in size and design depending on the desired size of the core and the size of the reactor.
When a new reactor starts, its core is often divided into a plurality, e.g., three or more groups of assemblies which can be distinguished by their position in the core and/or their enrichment level. For example, a first batch or region may be enriched to an isotopic content of 2.0% uranium-235. A second batch or region may be enriched to 2.5% uranium-235, and a third batch or region may be enriched to 3.5% uranium-235. After about 10 to 24 months of operation, the reactor is typically shut down, and the first fuel batch is removed and replaced by a new batch, usually of a higher level of enrichment (up to a preferred maximum level of enrichment). Subsequent cycles repeat this sequence at intervals in the range of from about 8 to 24 months. Refueling, as described above, is required because the reactor can operate as a nuclear device only so long as it remains a critical mass. Thus, nuclear reactors are provided with sufficient excess reactivity at the beginning of a fuel cycle to allow operation for a specified time period, usually between about 6 to 18 months.
Conventional fuel pellets for use in PWRs, for example, are typically fabricated by compressing suitable powders into a generally cylindrical mold. The compressed material is sintered, which results in a substantial reduction in volume. The resulting pellet is generally cylindrical and often has concave surfaces at each end as a result of the compression. The fuel pellets are typically composed of uranium dioxide. The uranium component of the uranium dioxide includes uranium-238 and uranium-235. Typically, the fuel composition of the pellets includes a large amount of uranium-238 and a small amount of uranium-235. For example, a conventional fuel pellet can include a maximum of less than 5% by weight of uranium-235 with the remainder of the uranium in the uranium component composed of uranium-238.
The percentage of uranium-235 in the fuel composition of the pellet can be increased as follows: (i) by using a greater percentage, e.g., greater than 5% by weight (which is currently the licensed limit for many nuclear fuel fabrication facilities), of uranium-235 in the fuel composition or (ii) by increasing the density of the fuel composition to allow for a larger amount of uranium-235. A higher percentage of uranium-235 in the fuel pellet composition can provide economic benefits, such as longer fuel cycles and/or the use of fewer new fuel assemblies during batch replacement of a region. Further, higher thermal conductivity, if it can be obtained, will enable higher thermal duty.
Thus, there is a need to increase the content of uranium-235 and to increase the thermal conductivity of uranium-containing fuel compositions.